1. Field of the Invention
The present invention relates to an apparatus and method for forming a thin film. More particularly, the invention relates to an apparatus and method for forming, on a plastic substrate, a thin film having improved scratch resistance, an medium refractive index, and strong adhesion to the substrate.
2. Background Art
Conventionally, a variety of techniques have been known for forming a hard coating film on a plastic substrate.
For example, an organic silicone coating film has been formed on a plastic lens by dipping to serve as a hard coating film. In order to adapt to a recent plastic lens per se having a high refractive index, a silicone coating film having a corresponding refractive index must be developed. Such a coating film is formed by use of an apparatus which enables provision of a specific refractive index in the formed coating film. Therefore, a facility for producing the coating film must disadvantageously be equipped with a plurality of dipping apparatus.
When a hard coating film is formed by use of a conventional dipping apparatus, a substrate must be subjected to a surface pretreatment, such as immersion in an alkaline solution. In view of environmental safety, the process requires an additional treatment, such as treatment of wastewater. In addition, a conventional method for forming a hard coating film requires a condensation-curing step requiring several hours. Thus, solving these problems is very important for saving time until product delivery.
In order to solve these problems, a variety of techniques are proposed. For example, Japanese Patent Application Laid-Open (kokai) No. 8-190002 discloses a method for coating an optical material substrate through plasma polymerization. The method comprises a step for controlling the refractive index of the coating film by controlling the composition of a polymerization atmosphere and/or controlling at least one process parameter for coating the substrate through plasma polymerization, such that the refractive index varies non-stepwise from the initial value at the top of the substrate to a final value at the top surface of the coating film.
Japanese Patent Application Laid-Open (kokai) No. 8-146358 discloses a method for coating. The method comprises transforming into gas, under reduced pressure in a chamber, an organic titanium compound having an alkoxyl group and an organic silicon compound having an alkoxyl group serving as monomers; introducing the gas into a chamber containing plasma under reduced pressure to thereby cause plasma reaction; depositing the thus-formed compounds on a substrate while modifying the refractive index of the formed thin film to thereby form an intermediate matching layer on the substrate; and forming a thin film comprising an organic silicon compound having an alkoxyl group on the matching layer. In the method, an anti-reflection film and an optional water-repellent thin film comprising an organic fluorocompound and/or an organic silicon compound are further formed.
Japanese Patent Application Laid-Open (kokai) No. 7-56001 discloses a hard coating film and a method for forming the film. Specifically, there are disclosed a hard coating layer formed on a plastic substrate wherein the refractive index at the portion contacting the plastic substrate is approximately equal to that of the plastic substrate and the refractive index gradually or stepwise varies with respect to the thickness direction, and an anti-reflection film formed on the hard coating layer. In addition, a method is disclosed for forming a hard coating layer and an anti-reflection layer on a plastic substrate so as to mechanically protect the plastic substrate and impart an anti-reflection function to the substrate. The method comprises forming a hard coating layer wherein the refractive index gradually or stepwise varies with respect to the thickness direction such that the refractive index at the portion contacting the plastic substrate is approximately equal to that of the plastic substrate and the top surface of the hard coating film has a predetermined refractive index for designing a coated product. The method further comprises forming on the hard coating layer an anti-reflection film which is designed on the basis of the predetermined refractive index.
Furthermore, a variety of other techniques are also proposed. For example, Japanese Patent Application Laid-Open (kokai) No. 56-14789 discloses a method for forming a hard coating film, which method comprises feeding an organic monomer or polymer into a chamber in which a plastic substrate is placed; ionizing the monomer or polymer by plasma to form an organic polymer layer on the plastic substrate; and vapor-depositing an inorganic hard substance while continuing formation of the polymer layer to thereby form an organic-inorganic composite layer. Japanese Patent Application Laid-Open (kokai) No. 56-147830 discloses a method for forming a hard coating film, which method comprises feeding an organic monomer or polymer into a chamber in which a plastic substrate is placed; ionizing the monomer or polymer by plasma to form an organic polymer layer on the plastic substrate; vapor-depositing an inorganic hard substance while continuing formation of the polymer layer to thereby form an organic-inorganic composite layer; and subsequently vapor-depositing an inorganic hard substance so as to form an inorganic hard layer.
As described above, there have been known techniques for forming an SiO.sub.2 or SiO.sub.2 composite film through direct plasma CVD, with products attaining high temperature due to employment of plasma; techniques for making the refractive index at the top surface of a thin film constant by use of a refractive index-graded film which matches any substrate; and techniques for forming a hard coating film through combination of CVD (plasma polymerization) and vapor deposition.
An SiO.sub.2 film having a refractive index of approximately 1.46 can be easily and reliably formed through any one of these methods. However, satisfactory, industrial-scale production of a film having a higher refractive index with a constant refractive index at the top surface of a thin film through plasma CVD or by use of a refractive index-graded film which matches any substrate has not been attained thus far, due to restriction of a production apparatus and fabrication of a production apparatus.
Specifically, when direct plasma CVD is employed, stable operation of a CVD apparatus while constant component proportions of gasified materials are maintained is difficult, due to difficulty in flow control of a monomer serving as a TiO.sub.2 source among two categories of gasified substances. In other words, the process requires long-term, stable operation in a production line and a control apparatus for stably gasifying a liquid or solid CVD source.
Low-temperature film formation is preferred during production of a hard coating film through combination of direct plasma CVD (plasma polymerization) and vapor deposition. In addition, when direct plasma CVD is employed, control of plasma must be considered in order to form a film at low temperature.